JPH03281050A - Production of continuously cast slab without center porosity - Google Patents

Abstract

PURPOSE:To prevent generation of center porosity and to favorably improve the internal quality of a continuously cast slab by applying larger rolling reduction than unsolidified thickness in this cast slab in the range, which solid phase ratio in the center part of thickness in the cast slab becomes the specific value or higher. CONSTITUTION:In the range, where the solid phase ratio in the center part of thickness in the continuously cast slab is >=0.9, the larger rolling reduction than the unsolidified thickness in this cast slab, is applied. By mutually approaching/separating dies 2a, 2b with rotation of a crank shaft 5, squeezing work is applied to the cast slab strand 1. It is suitable to use the dies having inclined surface O at tip part thereof from where the cast slab strand 1 is received so as not to develop crack on solidified interface in the cast slab strand 1 at the time of applying the squeezing work. As the center porosity exists in almost center part in the cast slab, in the case of continuously casting a bloom, the apparatus having such construction that the prescribed range of the bloom width in center range in width direction can be locally applied with the rolling reduction is favorable.

Description

[Detailed Description of the Invention] (Industrial Application Field) The center porosity that exists inside a continuously cast slab obtained by continuous casting is inevitably caused by volumetric contraction during the solidification process of steel. However, it is difficult to eliminate this by rolling, and when the continuously cast slab is charged into a heating furnace or the like without eliminating this, the inside of the continuously cast slab may be oxidized, causing a major quality problem. This invention relates to continuously cast slabs obtained by continuous casting of steel, especially high Cr steel and high Mn steel.
The purpose is to effectively prevent noticeable center porosity in continuously cast high alloy slabs such as steel, high Mo steel, etc.

(Prior art) Conventionally, attempts to reduce the center porosity that exists inside continuously cast slabs include methods such as low-temperature casting and electromagnetic stirring during continuous casting to increase equiaxed crystals (Japanese Patent Laid-Open No. 61
-229450), a method of lowering the casting speed, or a method of applying light reduction to continuously cast slabs obtained by continuous casting (see Japanese Patent Laid-Open No. 4912738).

However, although low-temperature casting and electromagnetic stirring can reduce center porosity, the reduction effect is very small, and methods that reduce the casting speed can reduce the diameter of center porosity, but the proportion of center porosity that occurs is small. In addition, in the case of the light reduction method, the reduction of center porosity was extremely small because the reduction blade could not be transmitted to the center of the slab.

(Problem to be solved by the invention) Obtain a continuously cast slab with good internal quality by eliminating center porosity, which is particularly noticeable in continuous casting of high alloy blooms such as high Cr steel, high Mn steel, or high MO steel. It is an object of the present invention to propose a new manufacturing method that can.

(Means for Solving the Problems) This invention provides for the continuous casting slab to be drawn out from a mold for continuous casting in a region where the solid phase ratio at the center of the thickness of the slab is 0.9 or more. A method for manufacturing a continuously cast slab without center porosity (first invention) characterized by applying a reduction larger than the solidification thickness, and this invention also relates to a method for manufacturing a continuously cast slab drawn from a continuous casting mold. A method for manufacturing a continuously cast slab without center porosity, characterized by applying a reduction of 15% or less of the thickness of the continuously cast slab immediately after completion of the process (Second method)
invention).

Here, the above-mentioned continuously cast slabs, especially high alloy continuously cast slabs, are as follows:
High Cr steel with Cr>1%, high Mn steel with Mn>1%, Mo
>0.05% high MO steel or C>0.7% medium/high carbon steel, such high alloy continuous cast slabs are
In conventional continuous casting, center porosity occurs significantly.

(Function) In order to eliminate the center porosity that exists inside a continuously cast slab, it is effective to apply pressure to the slab and deform its inside. When heating a slab, the temperature distribution in the cross section of the slab is uniform, or the temperature is higher than the outside or inside of the slab, so in such conditions, even if the continuously cast slab is reduced, the slab will not melt. Since the outer circumference was largely deformed and the internal deformation was small, the center porosity could not be eliminated, and therefore a very large rolling blade was required to eliminate the center porosity.

In this invention, since the reduction is applied during the drawing process of the slab during continuous casting, that is, when the temperature of the center of the slab is high and the temperature of the outer circumference is low, the center porosity can be effectively crimped. It is. Here, the solid phase ratio at the center of the thickness of the continuously cast slab (an index that indicates the temperature at the center of the slab or the position between the liquidus temperature and the solidus temperature determined by the steel type) The reason why we decided to apply a pressure larger than the unsolidified thickness in the region where the thickness is 0.9 or more is because this region is in a mixed state of solid phase and liquid phase.
At this point, there is no center porosity inside the slab, but a reduction greater than the unsolidified thickness (1.5 to 2.0 times or more of the unsolidified thickness) is applied in advance to form a resolidified part between the unsolidified parts. This is because by crimping, the occurrence of center porosity can be avoided in advance. In addition, in this invention, even after solidification of the continuously cast slab is completed, by applying a reduction of 15% or less of the thickness of the slab immediately after, even if center porosity occurs inside, it can be effectively removed. can be resolved. That is, since the inside of the slab is in a high temperature state immediately after solidification is completed, the center porosity inside can be compressed by applying a reduction including thickness reduction. Note that when pressing is performed immediately after solidification is completed, the temperature inside the slab may be 1100° C. or higher depending on the rolling blade. In order to press the center porosity, it is necessary to apply a reduction of 2% or more of the slab thickness, more preferably 596 or more, and to transmit this reduction blade to the inside. In this invention, the reason for applying a reduction of 1596 or less of the thickness of the slab is that if the reduction exceeds this, the crimp effect of the center porosity will be saturated, and the pressure required to reduce the solidified slab. This is for economical reasons, as increasing the size of the rolling down means increases the size of the rolling down means, and in the case of rolling down at this point or local rolling down, 15
This is because if the rolling is more than %, there is a risk that indentations or rolling scratches may occur in the subsequent rolling process.

Specific means for applying reduction to continuously cast slabs are as follows:
It may be possible to use rolls, but in the case of rolls, cracks may occur at the solidification interface inside the slab depending on the amount of reduction. For this reason, FIG. 1(a).

It is preferable to use a forging machine equipped with a mold having a flat forging surface as shown in (b).

In Figures 1(a) and (b) above, number 1 is a continuously cast slab;
2a and 2b are the molds that hold the continuously cast slabs on top and bottom, 3 is the mold 2
4 is movable forward and backward along the guide surface of the frame body 3, and has a hydraulic cylinder S at its lower end. Slider 5 with mold 2a is a crankshaft, frame main body 3 and slider 4 are suspended and supported by crankshaft 5 via links 6 and 7, respectively, and the mold 2a is rotated by rotation of crankshaft 5. , 2b are moved closer to each other and separated from each other, and the slab strand l is subjected to a forging process. Reference numeral 8 denotes a hydraulic cylinder that quickly restores the frame body 3 to its initial state, which has moved as the slab strand 1 is pulled out during forging. Figure 1(a)
is mold 2a.

FIG. 1 shows a state in which the molds 2b are separated and released from each other (bl shows a state in which the molds are brought close to each other and the forging process is performed.

In addition, the molds installed in the device with the above configuration are as follows:
It is possible to use a plate whose entire surface is flat, but in order to prevent cracks from occurring at the solidification interface of the slab strand 1 during the stamping process, the slab strand 1 should be placed oppositely as shown in Fig. 2. It is preferable to use a mold having a zero inclined surface at its tip.

In addition, it is known that center porosity usually exists almost at the center of the slab, so when bloom is continuously cast, the bloom is located in the center area in the width direction as shown in Figure 3 (a). When continuously casting slabs, the width ranges from 20 to 4096, as shown in Figure 3(b).
A structure that allows local reduction of a 0% area is advantageous in terms of reducing the number of reduction blades.

FIG. 4 shows a situation in which a mold having the above-mentioned configuration is applied and a slab strand 1 is subjected to forging by repeatedly approaching and separating the molds from each other.

In the figure, the dotted chain line shows the state of the mold during the rolling of the slab strand, and the solid line shows the state of the mold after a predetermined forging process has been completed. In this invention, even if the center porosity t occurs inside the slab strand 1, the above-mentioned 4.
In the situation shown in the figure, it is possible to carry out appropriate forging processing that transmits the forging effect to the inside, so defects such as center porosity can be effectively eliminated.

(Example) Blooms of the steel types shown in Table 1 with a thickness of 400 mm and a radius of 560 m were continuously cast under conditions of a molten steel heating degree of 20 to 30°C and a drawing speed of 0.4 to 0.55 m/min. At the same time, from the area of unsolidified thickness of 20 m of the bloom to immediately after the completion of solidification, a forging process was applied using a device of the type shown in Fig. 1 above, and each continuous cast slab obtained was We investigated the internal quality of The results are shown in Table-1.

When the forging process according to the present invention was applied, the existence of center porosity was not observed at all, but in the continuously cast slab manufactured according to the usual method without applying forging process during continuous casting, the cross section of the slab was It was confirmed that center porosity of 5 to 10% in terms of σ area ratio was observed.

(Effects of the Invention) Thus, according to the present invention, center porosity, which has been difficult to avoid in slabs obtained by continuous casting, can be fixed in a V shape without the need for a large reduction blade, and the interior of the continuously cast slab can be fixed. Quality can be improved advantageously.

[Brief explanation of the drawing]

Figure 1 (aXb) is a schematic diagram of a forging machine suitable for carrying out the present invention; Figure 2 is a schematic diagram of a forging die suitable for carrying out the present invention; ) is a sectional view of a main part of a die for forging, and FIG. 4 is an explanatory diagram of a forging procedure according to the present invention. I...Continuously cast slab 2a, 2b...Mold 3
...Frame body 4...Slider 5...
Crankshaft 6, 7...Link SIT S2・
...Hydraulic cylinder 0...Slanted surface @1 Fig. (a) (Fig. 2, Fig. 3)

Claims (1)

[Scope of Claims] 1. Greater than the unsolidified thickness of the continuously cast slab in a region where the solid phase ratio at the center of the thickness of the continuously cast slab pulled out from the continuous casting mold is 0.9 or more. A method for manufacturing continuously cast slabs without center porosity, characterized by applying reduction. 2. A method for producing a continuous cast slab without center porosity, which comprises applying a reduction of 15% or less of the thickness of the continuous cast slab immediately after solidification of the continuous cast slab pulled out from a continuous casting mold. .